This invention relates to geothermal energy in general and more particularly to an improved method of extracting geothermal energy in a hot, dry rock system.
With the shortage of petroleum products and high prices, there is a great deal of interest in alternate sources of energy. One such source is geothermal energy. This energy is energy taken from the natural heat of the earth. Various systems have been developed for such purposes. Typical are those disclosed in U.S. Pat. Nos. 3,187,038, 3,786,858 and 3,911,638. In a hot dry rock system such as that disclosed in U.S. Pat. No. 3,817,038, an injection well and a production well are drilled and a working fluid is injected into a geothermal area through the injection well, the fluid forces through the formation with simultaneous heating and the heated working fluid then recovered from the production well. The recovered heated fluid is then used on the surface to generate energy. For example, the heated working fluid may be expanded and the vapor separated and delivered to a steam turbine. The remaining liquid along with any makeup water then being reinjected in the injection well to form a closed system.
One of the major problems associated with hot dry rock systems is the adverse effects of using water as the heat extraction or working fluid.
Recent experiments have demonstrated that substantial silica and mineral dissolution takes place when either distilled water or sodium chloride brines come into contact with typical rock materials. The rate of interaction between rock material and the circulating fluid is significantly increased when the fluids contain sodium chloride. Dense coatings of alteration products form and marked swelling takes place within the rock material.
The presence of dissolved materials in the circulating working fluid will cause harmful scale formation when the hot water is flashed to steam. Of course, this can be mitigated to some degree through the use of an intermediate heat exchanger that heats a secondary working fluid such as isobutane or propane. However, the intermediate heat exchanger will still be susceptible to scaling problems and a two stage system will typically cost significantly more than a direct steam turbine system.
In view of this, the need for an improved working fluid which reduces these problems become evident.